We calculate the free energies of unstable stacking fault (USF) configurations on the glide and shuffle slip planes in silicon as a function of temperature, using the recently developed environment-dependent interatomic potential (EDIP). We employ the molecular dynamics (MD) adiabatic switching method with appropriate periodic boundary conditions and restrictions to atomic motion that guarantee stability and include volume relaxation of the USF configurations perpendicular to the slip plane. Our MD results using the EDIP model agree fairly well with earlier first-principles estimates for the transition from shuffle to glide plane dominance as a function of temperature. We use these results to make contact to brittle-ductile transition models. [S0163-1829(98)06043-3].58191255512558